1. Field of the Invention
The present invention relates to a reverse-conducting IGBT (RC-IGBT: Reverse-Conducting Insulated Gate Bipolar Transistor) forming characteristics of an IGBT and a diode with one structure, and in particular to a semiconductor device which can reduce snapback.
2. Background Art
In general, power devices have various demands such as voltage withstanding capacity, guarantee of a safety operation region for preventing an element from being destroyed during operation and the like, and one of the most important demands is low loss. Low loss of the power device has effects such as size reduction, weight reduction and the like of the device and leads to consideration to global environments through reduction of energy consumption in a broader meaning. Moreover, realization of these characteristics with a cost as low as possible is in demand. As one of means for solving this problem, a reverse-conducting IGBT forming characteristics of an IGBT and a diode with one structure is proposed.
The reverse-conducting IGBT is illustrated in p. 133 to 136, ISPSD2004. In a normal IGBT, only a p+-type collector layer is formed on a back surface, but in the reverse-conducting IGBT, a p+-type collector layer and an n+-type cathode layer is formed on a back surface. Ideally, the reverse-conducting IGBT can obtain two effects of an IGBT and a diode with one structure.
This reverse-conducting IGBT has some technical problems. One of them is snapback while conducting. In order to maintain a low forward voltage, it is necessary that a p-n junction formed of the p+-type collector layer and an n−-type drift layer is forward-biased and a positive hole is injected from the p+-type collector layer. However, due to the presence of the n+-type cathode layer, an electron current flowing from an emitter electrode flows to the n+-type cathode layer, and there is a problem that, until this junction is turned on, snapback occurs, while conductivity modulation does not occur, and an on voltage becomes high. A problem of the same nature also occurs during an FWD operation with a flow in an opposite direction. If a gate voltage becomes positive, and an n-channel MOSFET channel enters an accumulated state, the electron current flows to the channel side, and a similar snapback phenomenon occurs, which incurs an increase in loss.
In response to these problems, a structure in which a locally wide p+-type collector layer is made and an IGBT operation portion, first, is formed is illustrated in p. 283 to 286, ISPSD2009. Moreover, a structure in which an n+-type cathode layer is surrounded by a p-type separation layer, and an oxide film is embedded in a boundary with a back electrode is illustrated in p. 161 to 164, ISPSD2007. Both have an effect of suppressing snapback in an operation direction of an IGBT. However, the former has a problem of an increase in a chip area, and the latter has a problem of a rise of heat resistance of the back electrode and the like.
Another problem of the reverse-conducting IGBT is an increase of a recovery current during the FWD operation. In the reverse-conducting IGBT, there is a relationship of trade-off in a part of the characteristics of the IGBT operation and the FWD operation, respectively. Thus, when balance is considered, it is difficult to lower the recovery current. In order to solve this problem, there are methods of substantially lowering concentration of a p-type base layer, of macro arrangement of the IGBT/FWD, of local lifetime control and the like (see Japanese Patent Laid-Open No. 2005-101514, Japanese Patent Laid-Open No. 2005-317751, Japanese Patent Laid-Open No. 2007-134625, and Japanese Patent Laid-Open No. 2008-053648, for example). However, there is a problem of trade-off such as a lowered resistance amount, deterioration of area efficiency and the like, and sufficient improvement has not been made.